You don't need the original assembly (I don't have rights to share that), because a test assembly can be easily constructed as below,
1. Create a new WPF project in VS.
2. Add NuGet package "WPFToolkit" and "WPFToolkit.DataVisualization".
3. Add new UserControl. Its C# code should be similar to https://github.com/lextm/ILSpy/blob/306e7143b79780d8b529f507f417a10372ccb048/ILSpy.BamlDecompiler/Tests/Cases/issue365.xaml.cs
4. Modify the XAML to https://github.com/lextm/ILSpy/blob/306e7143b79780d8b529f507f417a10372ccb048/ILSpy.BamlDecompiler/Tests/Cases/issue365.xaml
5. Compile the project and then use JustDecompile to decompile the BAML file for this UserControl and you can see the exception.
The ColumnSeries tag is the key here to trigger the bug.